1. Field of the Invention
Embodiments of the invention relate to a display device, more particularly, to an array substrate for liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) includes a first substrate, a second substrate, and a liquid crystal layer. The first and second substrates face each other and are spaced apart from each other. The liquid crystal layer is interposed between the first and second substrates. The LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display images.
The liquid crystal molecules have orientation or arrangement characteristics resulting from their thin long shape. Such characteristics of the liquid crystal molecules can be controlled by applying an electric field. More particularly, the LCD device can include a thin film transistor (TFT) as a switching element to control application of the electric field. An LCD using a TFT is referred to as an active matrix LCD (AM-LCD) device, which has high resolution and smoothly displays moving images.
FIG. 1 is an exploded perspective view of a related art LCD device. As shown in FIG. 1, the LCD device includes first and second substrates 12 and 22, and a layer of liquid crystal molecules 30. The first and second substrates 12 and 22 face each other with the layer of liquid crystal molecules 30 interposed therebetween. The first substrate 12 includes a gate line 14, a data line 16, a thin film transistor (TFT) “T”, and a pixel electrode 18. The gate line 14 and the data line 16 cross each other such that a pixel region “P” is defined between the gate and data lines 14 and 16. The TFT “T” is formed adjacent to where the gate and data lines 14 and 16 cross each other. The pixel electrode 18 is formed within the pixel region “P” and connected to the TFT “T”.
The second substrate 22 includes a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 has a lattice shape that covers non-display regions of the first substrate 12, such as the gate line 14, the data line 16, and the TFT “T.” The color filter layer 26 includes first, second, and third sub-color filters 26a, 26b, and 26c. Each of the sub-color filters 26a, 26b, and 26c is respectively one of red, green, and blue colors “R”, “G”, and “B” and are respectively positioned in a pixel region “P”. The common electrode 28 is formed on the black matrix 25 and the color filter layers 26 so as to be over the entire surface of the second substrate 22. As mentioned above, the arrangement or orientation of the liquid crystal molecules is controlled by an electric field applied between the pixel electrode 18 and the common electrode 28 such that the amount of transmitted light is controlled. By using a plurality of pixels “P,” the LCD device displays images.
Although not shown in FIG. 1, a seal pattern may be formed along edges of the first and second substrates 12 and 22 to prevent the liquid crystal molecules 30 from leaking. First and second alignment layers may be formed between the first substrate 12 and the liquid crystal layer 30 as well as between the second substrate 22 and the liquid crystal layer 30. A polarizer can be formed on at least an outer surface of the first and second substrates 12 and 22.
The LCD device can also include a backlight assembly at an outer surface of the first substrate 12 to supply light to the liquid crystal layer 30. In this case, the polarizer can be positioned between the first substrate 12 and the backlight assembly. The light passes through the first substrate 12, the liquid crystal layer 30 and the second substrate 22 such that the LCD device displays images.
In the past, glass plates were used for the first and second substrates 12 and 22. However, a flexible plate, such as a plastic plate, can be used for the first and second substrates 12 and 22 because the flexible plate is much lighter and more flexible than a glass plate. Unfortunately, since a process of fabricating an array substrate is performed at a temperature higher than about 200° C., it is very difficult to substitute a flexible plate for a glass plate. Further, when the semiconductor layer is formed of one of amorphous silicon and polycrystalline silicon at a temperature lower than about 200° C., electrical characteristics of the TFT are unsatisfactory. To resolve these problems, a method of fabricating the array substrate at a temperature lower than about 200° C. using an organic semiconductor material to form the TFT on the flexible substrate has been suggested.
A metal material for electrodes, an insulating material and a passivation layer can be formed on the flexible substrate by one of low-temperature deposition and coating processes. These methods do not affect the electrical characteristics of the TFT. However, when the semiconductor layer is formed of amorphous silicon at a temperature lower than about 200° C., there are electrical conductivity problems. To overcome these problems, the semiconductor layer is formed of an organic semiconductor material instead of amorphous silicon.
FIG. 2 is a cross-sectional view showing an array substrate for an LCD device according to the related art. A source electrode 55, a drain electrode 57, a data line (not shown), a pixel electrode 60, an organic semiconductor layer 63 and a gate insulating layer 67 are formed on a first substrate 51, which is either a plastic plate or a glass plate. The source electrode 55, the drain electrode 57 and the data line are formed on the substrate 51 by depositing and patterning a metal layer (not shown). The source and drain electrodes 55 and 57 are separated from each other. The metal layer may include gold (Au) and have a high work function. The source electrode 55 extends from the data line.
A transparent conductive layer (not shown) is deposited on the source and drain electrodes 55 and 57. The transparent conductive layer is patterned such that the pixel electrode 60 is formed on the substrate 51. The pixel electrode 60 is connected to the drain electrode 57.
The organic semiconductor layer 63 is formed of an organic semiconductor material layer (not shown) using a shadow mask (not shown). The organic semiconductor material layer is either deposited or coated. The organic semiconductor layer 63 contacts the source and drain electrodes 55 and 57. The gate insulating layer 67 is formed both on the semiconductor layer 63 and the pixel electrode 60.
As mentioned above, the source and drain electrodes 55 and 57 and the data line are formed of gold because gold has a high work function and low resistance. The work function of the gold is about 5.1 eV. The organic semiconductor material includes one of pentacene and polythiophene. There is an energy barrier at the interface between the organic semiconductor layer of pentacene (or polythiophene) and the source electrode (or drain electrode). However, when the source and drain electrodes are formed of a metal having a high work function, the energy barrier is reduced. Thus, the energy barrier between the organic semiconductor layer and the source electrode and between the organic semiconductor layer and the drain electrode is reduced by forming the source and drain electrodes from gold, which has a high work function, so as to improve the electrical characteristics of the TFT. However, when the source and drain electrodes are formed of gold, it is difficult to control widths of the source and drain electrode because gold diffuses easily. Moreover, when the gold is deposited in the chamber, the gold is deposited on the wall of the chamber. The deposited gold can subsequently drop onto the substrate during the process so as to deteriorate the LCD device. Further, production costs are increased because of the gold being wasted on the walls of the chamber.